Borzov Egor, Nevelsky Alexander, Bar-Deroma Raquel, Orion Itzhak
Department of Radiotherapy, Division of Oncology, Rambam Health Care Campus, Haifa, Israel.
Department of Nuclear Engineering, Ben-Gurion University of the Negev, Beer Sheva, Israel.
J Appl Clin Med Phys. 2018 Jan;19(1):194-203. doi: 10.1002/acm2.12242. Epub 2017 Dec 20.
Dosimetry of small fields defined by stereotactic cones remains a challenging task. In this work, we report the results of commissioning measurements for the new Elekta stereotactic conical collimator system attached to the Elekta VersaHD linac and present the comparison between the measured and Monte Carlo (MC) calculated data for the 6 MV FFF beam. In addition, relative output factor (ROF) dependence on the stereotactic cone aperture variation was studied and penumbra comparison for small MLC-based and cone-based fields was performed.
Cones with nominal diameters of 15 mm, 12.5 mm, 10 mm, 7.5 mm, and 5 mm were employed in our study. Percentage depth dose (PDD), off-axis ratios (OAR), and ROF were measured using a stereotactic field diode (SFD). BEAMnrc code was used for MC simulations.
MC calculated and measured PDDs for all cones agreed within 1%/0.5 mm, and OAR profiles agreed within 1%/0.5 mm. ROF obtained from the measurements and MC calculations agreed within 2% for all cone sizes. Small-field correction factors for the SFD detector K (SFD) were derived using MC calculations as a baseline and were found to be 0.982, 0.992, 0.997, 1.015, and 1.017 for the 5, 7.5, 10, 12.5, and 15-mm cones respectively. The difference in ROF was about 10%, 6%, 3.5%, 3%, 2.5%, and 2% for ±0.3 mm variations in 5, 7.5, 10, 12.5, and 15-mm cone aperture respectively. In case of single static field, cone-based collimation produced a sharper penumbra compared to the MLC-based.
Accurate MC simulation can be an effective tool for verification of dosimetric measurements of small fields. Due to the very high sensitivity of output factors on the cone diameter, manufacture-related variations in cone size may lead to considerable variations in dosimetric characteristics of stereotactic cones.
立体定向锥形准直器所定义的小射野剂量计算仍是一项具有挑战性的任务。在本研究中，我们报告了安装在医科达VersaHD直线加速器上的新型医科达立体定向锥形准直器系统的调试测量结果，并给出了6 MV FFF射束的测量数据与蒙特卡罗（MC）计算数据的比较。此外，研究了相对输出因子（ROF）对立体定向锥形准直器孔径变化的依赖性，并对基于多叶准直器（MLC）的小射野和基于锥形准直器的小射野的半值层进行了比较。
本研究采用标称直径为15 mm、12.5 mm、10 mm、7.5 mm和5 mm的锥形准直器。使用立体定向射野二极管（SFD）测量百分深度剂量（PDD）、离轴比（OAR）和ROF。BEAMnrc代码用于MC模拟。
所有锥形准直器的MC计算PDD与测量值在1%/0.5 mm范围内一致，OAR曲线在1%/0.5 mm范围内一致。所有锥形尺寸下，测量得到的ROF与MC计算值在2%范围内一致。以MC计算为基线，得出SFD探测器K（SFD）的小射野校正因子，5、7.5、10、12.5和15 mm锥形准直器的校正因子分别为0.982、0.992、0.997、1.015和1.017。5、7.5、10、12.5和15 mm锥形准直器孔径分别变化±0.3 mm时，ROF的差异分别约为10%、6%、3.5%、3%、2.5%和2%。在单静态射野情况下，基于锥形准直器的准直方式产生的半值层比基于MLC的更清晰。
精确的MC模拟可作为验证小射野剂量测量的有效工具。由于输出因子对锥形准直器直径非常敏感，与制造相关锥形准直器尺寸的变化可能导致立体定向锥形准直器剂量学特性的显著变化。
